The present invention relates to the containment of thin wire filaments in a supportive aerogel matrix to reduce tension and stresses on the filaments, particularly for use in radiation measurement instrumentation, vacuum gauges, and incandescent lamps.
Ionization counters measure ionizing radiation by detecting the amount of charge liberated by the interaction of ionizing radiation with suitable gases, liquids, or solids. A conventional proportional counter is composed of a thin, conducting wire filament (anode) under tension that is insulated from and surrounded by a coaxial, cylindrical conducting electrode (cathode). A gas, usually a mixture of a noble gas and a halogen or organic gas, serves as an ionizing medium and fills the space between the electrodes in a gas tight chamber. High voltage is applied to the electrodes, creating a large electric field near the anode.
Ionizing radiation, such as x-rays, .beta.-rays, and .gamma.-rays, enters the gas tight chamber and ionizes the atoms in the gas, and the electric field sweeps the liberated electrons and positive ions out of the gas. The electrons drift toward the anode wire, causing an electron avalanche at the anode and a small electrical current pulse. The output signal is typically amplified by a factor of 10.sup.3 to 10.sup.8 and is proportional to the number of electrons released by the ionizing radiation.
The principle of the ionization counter has been extended to drift chambers, commonly used in high energy physics experiments to track energetic electrons and nuclear particles. A drift chamber is a collection of individual ionization counters, often numbering in the tens of thousands, and arranged in planar or cylindrical arrays. The configuration of the drift chamber is such that the failure of a single anode wire can cause an electrical short across many other wires in the array, so that a significant segment of the drift chamber becomes inoperative.
The anode wires are often made of gold-plated tungsten and are typically 20-30 microns in diameter. The wires are stretched and placed under a load of about 80% of their breaking strength. The largest drift chambers use anode wires up to six meters in length. The physical demands placed on the wires invite breakage or separation of the bond connecting the wire to the rest of the circuitry. The stresses on the wires are compounded by the strong electric fields applied to them.
A design concept that helps to support and isolate the anode is described in U.S. Pat. No. 5,003,177 by Hornstra (Mar. 26, 1991). A helically-shaped insulator surrounds an inner electrode over the entire length of a coaxial cable. The insulator maintains a constant distance between the electrodes and contains a screening electrode that prevents leakage currents from flowing from the high voltage outer electrode to the inner signal electrode. The width of the insulator is minimized to fill as little space as possible between the electrodes because this proportion is lost as ionization space. The present invention provides an insulating, gas-permeable spacer and support structure for thin, tightly strung filaments in a variety of devices.
Ionization chambers, and other devices with numerous or precariously positioned wires under tension, would have increased lifetimes if the stresses on the wires could be decreased by embedding the wires in a supportive matrix. A supportive covering on the filaments would reduce the incidence of wire failure and, if there is wire breakage, would limit the subsequent damage done to the surrounding array by the broken wires. The challenge lies in finding a supporting material that exhibits a variety of physical properties; the material must be electrically nonconductive, thermally and mechanically stable, extremely gas-permeable, and transparent to ionizing radiation.